Effects of post-deposition rapid thermal annealing on aluminum-doped ZnO thin films grown by atomic layer deposition

Aluminum-doped ZnO (AZO) thin films with alternating stacks of ZnO and Al2O3 were grown by atomic layer deposition (ALD) on c-face sapphire substrates. Post-deposition rapid thermal annealing (RTA) at 950 °C for 5 min was conducted in all AZO samples. The X-ray diffraction patterns demonstrate that the intensity of cubic ZnAl2O4 (3 1 1) peaks grow with the increase of Al content, which implies the diffusion of Al atoms into ZnO. The reduction of dominant peak intensity of cubic ZnO (1 1 1) and the increase of hexagonal ZnO (1 0 0) and (1 0 1) peaks intensity suggest that there were variations of crystal structures for the samples with Al content above 6%. Two orders of magnitude of electron concentration raises in samples with 2 and 4% Al content compared with the as-grown without RTA-treated samples. It also infers that RTA facilitates diffusion of Al atoms in AZO material structures and activation of Al dopants. When Al content is above 6%, the variations of crystal structures with the enhancement of biaxial compressive strain result in blue shift in energy of photoluminescence peak and in frequency of image phonon mode of micro-Raman spectra. The deterioration of crystal quality due to the increase of strain-induced defects hinders the electrical and optical performance when Al doping concentration is above 6% in AZO materials.